The possible effect of Croton zambesicus administration on vital organs has been less investigated despite its extensive traditional use in tropical Africa. We therefore aim at elucidating the effect of ethanolic extract on the testes. The aqueous fraction of ethanolic leaf extract of C. zambesicus (5 and 10 mg/Kg body weight) was administered to verify its effect on sperm concentration, sperm motility, sperm progessivity, malondialdehyde and catalase activities for a period of five consecutive days. The result showed that there is a significant increase in sperm production, sperm motility and sperm progressivity in the treated group when compared with the control; while there was a reduction in malondialdehyde and catalase activity in all the treated groups. The slight increase in the weight of the measured parameters also indicated the positive effect of the extract in the normal metabolic activities in the treated groups. This investigation has shown that the leaf extract possesses promising profertility property which can be exploited in fertility therapy.Key words: Testes, Croton zambesicus, sperm production, sperm motility, sperm progressivity, malondialdehyd